1. Technical Field
The present invention relates to an electric motor system and a motor control method, and more particularly, to an electric motor system employing a brushless direct-current motor (BLDC) and a method for controlling operation of a BLDC motor.
2. Background Art
In an electrophotographic image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of those imaging functions, various pieces of motor-driven imaging equipment work in coordination with each other to perform a sequential electrophotographic process.
Different types of electric motors are employed in imaging applications depending on their own merits. Among these, brushless direct-current (BLDC) motors with electronic control circuitry increasingly replace stepper motors and conventional brushed motors owing to their high energy efficiency and light-weight construction. In particular, the brushless configuration of the BLDC motor is superior to its brushed counterpart in terms of durability due to the absence of wear and tear caused by friction with a commutator brush.
Some BLDC motor systems incorporate a feedback controller including a position sensor, such as a rotary encoder, for measuring an actual position and speed of the motor. Such a position sensor may be disposed directly on an output shaft of the motor, or otherwise on a load device connected to the motor shaft. Providing the position sensor on the motor shaft, as opposed to that on the load device, allows for a simple control circuit, as it does not require a complicated model accounting for displacement of the load device per revolution of the motor shaft. Also, the shaft-mounted sensor eliminates the need for modifications to the control circuitry where changes are made to the power transmission and the load device, or where a single motor is adapted from one place to another in the imaging equipment.
One problem associated with feedback control circuitry in a BLDC motor system is the difficulty in stabilizing operation of the motor in a hold state thereof in which the motor stops rotation while being energized. The problem arises where the feedback controller, detecting a difference between the targeted and measured positions of the motor shaft upon entry of the motor into the hold state, performs a corrective action in an attempt to reduce the positional error, resulting in oscillatory movement of the motor moving back and forth a slight distance corresponding to a number of pulses by which the measured position signal is shifted from the targeted position signal.
Oscillations of the motor would adversely affect coordination between motor-driven moving parts, such as those in the paper conveyance mechanism, as they propagate throughout the surrounding structure of the image forming apparatus. Also, irregular movement of the motor in the hold state causes accelerated wear and tear on the load device connected to the motor shaft, resulting in deteriorated performance of the imaging equipment.
To date, various techniques have been proposed to provide a reliable electric motor system for use in an image forming apparatus.
For example, one such technique employs a proportional, integral, and derivative (PID) controller that directs a driver circuit to supply an electric current to a BLDC motor. According to this method, the controller adjusts a gain of PID control depending on a difference between a reference pulse signal and a driving pulse signal representing desired and actual rotational speeds of the motor, respectively.
Another technique provides a dual control system for a BLDC motor, employing a combination of a PID controller and a sliding-mode controller, which can selectively perform PID control and sliding-mode control depending on a rotational speed of a load device being driven with the motor driver. According to this method, the PID controller is provided with a gain adjustment capability which reduces a gain of PID control progressively toward zero in response to switching of the control mode from the PID control to the sliding-mode control.
Although generally successful for their intended purposes, the techniques described above do not effectively stabilize operation of a BLDC motor in the hold state. That is, adjusting a controller gain depending on a differential speed, though effective for optimizing the controller gain to control the motor in the steady operational state, does not address the problem of motor oscillations in the hold state. Also, the dual control system cannot immediately stop oscillations of the motor in the hold state, where it takes time to attenuate the gain to zero after the control mode switches from the PID control to the sliding-mode control.